Satellite broadcasting systems such as a DVB-S2 system will be taken as an example, but the present invention is not limited to them.
In recent years, as the demand for high-quality, high-speed satellite broadcasting services are increasing, an adaptive modulation and coding scheme is applied for efficiently using channels and maximizing transmission capacity in the satellite broadcasting systems.
In the case of the adaptive modulation and decoding scheme, lengths of transmitted frames are different according to modulation schemes. Therefore, a receiver must acquire information about transmitted frame structures in order to perform estimation and correction processes using pilot or the like.
According to a conventional method for determining a frame structure, information about the transmitted frame structure is channel-coded in a header of the frame and then is transmitted, and a receiver determines the transmitted frame structure by demodulating and decoding the information about the transmitted frame structure. The satellite broadcasting system such as DVB-S2 system, however, has a limitation in that it cannot meet the requirement of a decoding performance for the information about the frame structure in an initial synchronization mode in which a frequency synchronization is not acquired because of signal distortion caused by low signal-to-noise ratio (SNR) and great frequency offset.
More specifically, the DVB-S2 system is one of satellite broadcasting systems and uses an adaptive modulation and coding scheme in order to overcome bad channel conditions caused by rain attenuation and guarantee transmission capacity. Also, the DVB-S2 system adaptively varies number of bits per frame and selectively inserts pilot sequence, depending on channel state and change in an amount of transmission data.
A receiver of a DVB-S2 broadband adaptive satellite broadcasting system requires a PLSC decoding to determine the transmitted frame structure. The PLSC is (64, 7) codeword in which 7 bits are encoded. The PLSC contains information about modulation, coding, number of bits per frame, and existence/nonexistence of pilot symbol.
The DVB-S2 system considers the frequency offset of −5 MHz to +5 MHz as well as the low SNR. Thus, assuming that the bandwidth is 25 MHz, the frequency offset corresponds to 20% of the bandwidth. Therefore, in an initial synchronization mode in which the frequency estimation and correction are not achieved, the PLSC detection and decoding performance is unsatisfactory due to the performance degradation caused by the frequency offset, even when the symbol synchronization and the frame synchronization are acquired. Moreover, because there is no information about the frame structure when the PLSC decoding is not achieved, fine adjustments such as frequency estimation are impossible.
In other words, in order to identify the frame structure, the conventional adaptive communication system identifies the transmission format by demodulating and decoding a transmission format identifier that is a symbol sequence in which information about the transmission format such as PLSC is described. However, if the adaptive communication scheme is applied to the satellite communication system, such as the DVB-S2 system, in which the frequency offset corresponding to 20% of the bandwidth exists, the PLSC demodulation and decoding performance is degraded by the frequency offset. Consequently, it is difficult to obtain the required performance.